Raman scattering and infrared reflectivity study of orthorhombic/monoclinic LaTiNbO6 microwave dielectric ceramics by A/B-site substitution

The structure origin of microwave dielectric properties in A/B-site substituted LaTiNbO6 ceramics with different crystal structures was explored by means of Raman scattering and infrared reflection spectra. Compared with the monoclinic (M) phase, the broadening of the Raman modes associated with B-O stretching and bending vibrations in the orthorhombic (O) phase was believed to be a result of the increased octahedral distortion. The Raman mode at 659 cm−1 was assigned to the stretching vibration of B-O-B and O-B-O bonds in the unique interlayer chain structure of O phase. The Raman modes of the BO6 tilting within 340-520 cm−1 suggest that different octahedral connection ways should be one of the vital reasons why M and O phases own opposite-sign temperature coefficient of resonance frequency τf. Moreover, the result of infrared reflection spectrum fitted with a four-parameter semiquantum model indicates that the dielectric response of both M and O phases would mainly originate from the A-BO6 external vibrations in the far-infrared frequency, particularly their AO8 structure units. These results would provide useful insights into the structure-property relation in the RETi(Nb,Ta)O6 (RE: rare earth) material system.